As component densities in integrated circuit technology continue to increase, it becomes vitally important to possess an effective means of detecting the presence of sub-micron (i.e. less than 10.sup.-6 m) particulate contaminants on both patterned and unpatterned surfaces. The length scale of the patterned lines which comprise the basic elements of modern integrated circuits is rapidly approaching a level such that a particle which is on the order of 0.2 .mu.m will completely disrupt the operation of the circuit. With today's technology, particulate detection at the 0.1 .mu.m level is adequate for most purposes.
Known techniques for the detection of particulates on the surface of semiconductor wafer samples consist of either imaging or non-imaging techniques. In general, the imaging methods detect the presence of defects on patterned wafers by comparing the diffraction pattern of the image to a known diffraction pattern using a Fourier lens assembly. Defects appear as the differences between the known pattern and the diffraction pattern from the sample. These methods, however, are only useful for detecting contaminants on patterned surfaces. The nonimaging methods construct surface "images" by measuring some characteristic of the sample surface (e.g. the average surface roughness). Because these methods do not image the sample directly, they are primarily used in the characterization of unpatterned samples.
U.S. Pat. No. 5,189,481 describes a method and apparatus for detecting particulate contaminants on an unpatterned surface using a Coblentz sphere light detection assembly. In this method, light incident from the laser is focused on the sample surface. If there are particulate contaminants on the surface of the wafer, the incident photons will be diffusely reflected. The diffusely reflected light is then collected by the Coblentz sphere assembly and focused onto a detector. The intensity of the light on the detector is proportional to the number of particulate contaminants in the probe beam, or the total integrated scatter from the surface of the material. This method is not an imaging technique, so it cannot discriminate between a particulate contaminant and a rough sample surface.
U.S. Pat. No. 4,334,780 describes a method and apparatus for measuring the average surface roughness on an unpatterned surface by taking advantage of the diffusely reflecting properties of a "Gaussian rough surface". In this method, a collimated beam of light is reflected from a Gaussian rough surface and the spatial intensity distribution of the beam is subsequently measured by an imaging detector. The spatial distribution of the reflected beam can be associated with the average surface roughness of an ideal Gaussian reflecting surface. This method is not an imaging technique, so it cannot discriminate between a particulate contaminant and a rough sample surface. In addition, it cannot be used on patterned samples.
U.S. Pat. No. 3,614,232 describes a method for imaging defects in a patterned surface through the use of spatial filters. In this method, the incident beam of light is passed through a photomask which possesses the "defect free" diffraction pattern of an ideal patterned surface. When the photomask and the pattern on the sample are aligned properly, the only light which can reach the detector comes from defects in the pattern on the surface of the sample. In this way, defects on a patterned surface can be easily imaged. This method, however, requires that the diffraction pattern of the patterned surface be known, and that there be precise alignment between the photomask and the sample. In addition, this method of defect detection can only be used on patterned surfaces.
Various other U.S. patents use the diffraction properties of a patterned surface, and a unique spatial filter, to image defects on a sample surface. U.S. Pat. No. 4,330,775, for example, describes an apparatus which uses the differences in the diffraction patterns of a photomask and a patterned sample to detect defects on the sample. In addition, the sample is illuminated with incoherent light so that the image of the sample may be obtained simultaneously. U.S. Pat. Nos. 4,299,443, 4,806,774, 5,155,372, 5,172,000, 5,506,676, 5,659,390, and 5,719,405 all describe methods and instruments which detect defects in a repetitive pattern on a sample surface through the use of specific spatial filters matched to the pattern. All of these methods require the use of a unique spatial filter to detect defects in the unique pattern on the sample, and cannot be used to image particulates on unpatterned samples.
In short, the known methods of particle detection suffer from one or more of the following disadvantages: they are limited to the detection of particles on patterned surfaces, they are not sensitive to sub-micron size particles, they cannot discriminate between particulate contamination and surface roughness, and/or they require precise knowledge and matching of a surface pattern with a mask pattern,